The use of quartz crystal microbalances (also known as piezoelectric oscillators) in immunoassays has been described previously. These devices consist of single crystal wafers sandwiched between two electrodes. The electrodes are provided with means for connecting these devices to an external oscillator circuit that drives the quartz crystal at its resonant frequency. This frequency is dependent on the mass of the crystal, as well as the mass of any layers confined to the electrode areas of the crystal. Thus, the frequency is altered by changes in mass on the surface of the electrodes or in any layers on those electrodes. In general, the change in resonant frequency of these devices can be correlated to the amount of mass change; if the quartz crystal microbalance and any attached layers obey rigid-layer behavior, the mass change can be determined from the frequency change by the Sauerbrey relationship ##EQU1## where .DELTA.f is the measured frequency shift f.sub.0 the parent frequency of the quartz crystal, .DELTA.m the mass change, A the piezoelectrically active area, p.sub.q the density of quartz (2.648 g cm.sup.-3) and u.sub.q the shear modulus (2.947.times.10.sup.11 dynes cm.sup.-2 for AT-cut quartz).
Shons et al. describe a piezoelectric quartz crystal microbalance which has been modified for the determination of antibody activity in solution. A quartz crystal, precoated with antigen, is exposed to antisera of varying concentration and specificity. Antisera specific for the antigen coating will form an additional protein layer on the crystal. The thickness of the antibody layer, measured by the frequency shift of the dry crystal, is proportional to the concentration of specific antibody in solution. [J. Biomed. Mater. Res., Vol 6, pp. 565-570 (1972)].
U.S. Pat. No. 4,235,983, issued to Rice on Dec. 2, 1980, discloses a method for the determination of a particular subclass of antibody. The method utilizes a piezoelectric oscillator having bound to its surface an antigen specific for the antibody to be determined. The antigen-coated oscillator is exposed to a solution containing an unknown amount of the antibody. After the antibody in the solution is attached to the antigen on the oscillator, the oscillator is exposed to a so-called sandwiching substance which selectively binds to a specific subclass of the antibody being determined. The frequency of the oscillator is measured in the dry state before and after exposure to the sandwiching substance. The change in frequency is related to the amount of the subclass of antibody bound to the oscillator, and the amount of the subclass of antibody in the solution can be determined by reference to a standard curve.
Roederer et al. disclose an in-situ immunoassay using piezoelectric quartz crystals, specifically, surface acoustic wave devices. Goat anti-human IgG was immobilized on the quartz crystal surface with a coupling agent. The piezoelectric crystals were then placed in an electric oscillator circuit and tested for detection of the antigen human IgG. Detection was based upon the fact that surface mass changes by adsorption are reflected as shifts in the resonant frequencies of the crystals The authors concluded that the method suffers from both poor sensitivity and poor detection limits. The authors also concluded that the antigen to be detected must be of high molecular weight; low molecular weight analytes cannot be directly detected by this methodology. [Analytical Chemistry, Vol. 55, (1983)].
Ngeh-Ngwainbi et al. describe the use of piezoelectric quartz crystals coated with antibodies against parathion which are used for the assay of parathion in the gas phase. When the coated antibody binds with parathion by a direct reaction in the gas phase, the resulting mass change on the crystal generates a frequency shift proportional to the concentration of the pesticide. [J. Mat. Chem. Soc., Vol. 108, pp. 5444-5447 (1986)].
European patent application 0 215 669, published Mar. 25, 1987, discloses an analytical device and method for the in-situ analysis of biochemicals, microbes and cells. Again, the method is predicated on a resonant frequency change caused by a weight change on the surface of a piezoelectric crystal on which are immobilized receptor materials specific for the analyte to be detected.
Grabbe et al. describe a quartz crystal resonator, used in conjunction with cyclic voltammetry, to study the binding of human IgG and anti-IgG at a silver electrode. [G. Electroanal. Chem Vol 223, pp. 67-78 (1987)].
As discussed by Roederer et al., piezoelectric crystal-based immunoassays in which mass change is attributable only to the immunological reaction between an antigen and an antibody can, under certain circumstances, suffer from poor sensitivity and poor detection limit. Consequently there is a need in the art for a piezoelectric crystal-based specific binding assay in which the reaction between a binding agent and its ligand can be amplified to provide a more sensitive and reliable assay.